1. Field of the Invention
The invention relates to on-line control of the output power of a molecular fluorine laser beam, and particularly to a technique for redirecting VUV light of the beam to a VUV detector, while filtering visible light from the redirected beam.
2. Discussion of the Related Art
The molecular fluorine laser emitting at 157 nm has an advantageously short wavelength, or high photon energy. Because of this, very small structures, such as sub-0.18 micron structures and even sub-0.10 micron structures, may be formed by photolithographic exposure on semiconductor substrates. TFT annealing and micro-machining applications may also be performed advantageously at this wavelength.
For the applications mentioned above, on-line monitoring and control of the output power of the laser may be advantageously performed such that the energy stability of the output beam and overall performance of the laser may be enhanced. For this purpose, an energy or power detector may be configured to receive a split off portion of the output beam. The input voltage and other conditions such as the gas mixture composition may be actively adjusted depending on the measured pulse energy, energy dose or moving average energy in order to provide high stability.
There are several factors inhibiting use of conventional light detectors for on-line monitoring of VUV laser output. First, laser radiation below 200 nm is strongly absorbed in the atmosphere, e.g., by water vapor, oxygen, hydrocarbons, and fluorocarbons. Specifically, at 157 nm, the extinction length of a molecular fluorine laser beam is around 1 mm or less in ambient air due mostly to the presence of oxygen and water vapor in the air. Second, contaminants such as oil vapors and other organic substances generated, for instance, by vacuum pumps and plastic enclosures tend to form films on optical surfaces causing strong absorption. Third, the molecular fluorine laser generates, in addition to 157 nm light, radiation in the red part of the visible spectrum, between 600 and 800 nm, due to emission by excited atomic fluorine species in the laser gas mixture. This red emission is sensed by most optical detectors whose sensitivity tends to be higher in the visible part of the spectrum, as compared to that in the VUV range, i.e., at 157 nm.
It is therefore an object of the invention to provide a method and apparatus for detecting output power of a molecular fluorine laser beam without the beam being substantially absorbed as it propagates to the detector.
It is a further object of the invention to provide a method and apparatus for detecting the VUV output of a molecular fluorine laser while any accompanying visible output of the laser is substantially suppressed before reaching the detector.
In accord with the above objects, a beam delivery system for a laser emitting at a relevant wavelength of less than 200 nm is provided. The system includes a sealed enclosure surrounding the path of the beam as it exits the laser resonator. The enclosure extends between the laser output coupler and a photodetector sensitive at the wavelength of the relevant laser emission. The interior of the enclosure, and thus the beam path between the output coupler and the detector, is substantially free of species that strongly photoabsorb radiation at the relevant laser emission wavelength. A beam splitting element diverts at least a portion of the beam for measurement by the detector.
The beam splitting element preferably includes a beam splitting mirror, holographic beam sampler or diffraction grating. In addition, optics are preferably provided for filtering a visible portion of the diverted beam, so that substantially only a VUV portion of the diverted beam is received at the detector. The filtering optics preferably include a diffraction grating, holographic beam sampler or dichroic mirrors.